Spanning Tree Protocol (STP)

Spanning Tree Protocol (STP)

Spanning Tree Protocol (STP) was developed to prevent the broadcast storms caused by switching loops. STP was originally defined in IEEE 802.1D.
Switches running STP will build a map or topology of the entire switching network. STP will identify if there are any loops, and then disable or block as many ports as necessary to eliminate all loops in the topology.
A blocked port can be reactivated if another port goes down. This allows STP to maintain redundancy and fault-tolerance. 
However, because ports are blocked to eliminate loops, STP does not support load balancing unless an EtherChannel is used. EtherChannel is covered in great detail in another guide.
STP switches exchange Bridge Protocol Data Units (BPDU’s) to build the topology database. BPDU’s are forwarded out all ports every two seconds,to a dedicated MAC multicast address of 0180.c200.0000. 

Building the STP topology is a multistep convergence process: 
• A Root Bridge is elected
• Root ports are identified
• Designated ports are identified

• Ports are placed in a blocking state as required, to eliminate loops
The Root Bridge serves as the central reference point for the STP topology. STP was originally developed when Layer-2 bridges were still prevalent, and thus the term Root Bridge is still used for nostalgic reasons. It is also acceptable to use the term Root Switch, though this is less common. 
Once the full topology is determined, and loops are eliminated, the switches are considered converged. 

STP is enabled by default on all Cisco switches, for all VLANs.